Electronic Circuits and Projects Forum

Originally Posted by transistor495

Dual conversion FM receivers use 10.7(or more) first IF and 455KHz second IF to achieve better selectivity. Still with hi-fi audio.

Sorry, complete nonsense - dual conversion is for communication receivers, the reason for it is to give low bandwidth voice quality only.

A 455KHz IF is FAR too low to give decent bandwidth or HiFi quality.

AG seems to have his own warped opinion about anything that isn't full HiFi quality, and while communication receivers can give excellent voice quality (which is their reason for being) HiFi it isn't, and doesn't want to be.

Coincidently, there's no 'magic' reason for 10.7MHz IF - it's simple a compromise giving reasonable selectivity, reasonable bandwidth, at a reasonable cost and ease of manufacture.

With modern filters (rather than pre-WWII transformers) you can make receivers that greatly exceed the performance of 10.7MHz IF receivers, in every aspect. Many more modern communication receivers get their greatly improved performances from far higher IF frequencies.

I was never an amateur radio HAM.
In North America local telephone calls are free for any duration and sound better or the same as narrow band radio so I never used radio communications except for cell phones and FRS walkie-talkies.

Originally Posted by transistor495

G'day guys!

I've carried out some quick experiments after the arrival of my new RF Signal generator/counter. This is one of that. A transistorized FM superhet receiver which can demodulate and output broadcast band FM stations.

But the interesting thing is that I have started with a basic Medium Wave receiver circuit with 455KHz two stage IF and basic germanium diode AM detector. This one was a big success and I could receive the local AM stations clearly and apparently I got surprised a bit on how beautiful is the so called 'superheterodyne' principle!

The next stage was to remove the oscillator/mixer and detector part from the working receiver to actually convert it to an FM receiver. So I've added a simple charge pump FM demodulator consisting of 2 BC547's and a BF494 wired before the IF stages as a simplest mixer. Being connected with just a 5" antenna at the emitter of the transistor and Signal Generator output injected to the base in place of LO, let me say I'm quite surprised that the system could clearly receive couple of strong local FM stations nearby.

For 102.3MHz, I've injected around 102.9MHz to tune in the station clearly.

No tuned input circuits employed as of now and the whole circuit is powered using 3V(2xAA cells)

Next part would be to add an RF amp and LO so that to achieve a stand alone working FM receiver and I feel pretty confident that that would be so simple actually!

Below is the current status on breadboard, and you can see the signal generator outputting 102.9MHz used for ~455KHz down-converting for the receiver.

Cheers!

Attachment 60034

would you please post the schematic?


Here is what might be interesting for you too:

http://philsvalveradiosite.co.uk/phi...receiver_1.htm

http://www.vk2zay.net/article/250

http://www.youtube.com/watch?v=1q1NVZNNuiA

A breadboard is usually not used for radio circuits because its stray capacitance between the many rows of contacts and between all the wires is far too much. The many long wires cause interference within the circuit and they pickup all kinds of external interference.
For many years, 10.7MHz crystal filters have been used in the IF of an FM radio instead of 10.7MHz transformers.